Equalization circuit

ABSTRACT

A frequency equalization circuit includes an active filter circuit. The filter circuit differentiates the input signal. A plurality of adjustable resistances are connected to the differentiator corresponding to a plurality of operating or frequency ranges. A switch selects one resistance in accordance with one selected operating range to connect the filter circuit to an amplifier.

BACKGROUND OF THE INVENTION

There are a number of applications where a high frequency equalizationcircuit is needed to provide a signal rising or falling in amplitudewith frequency over a plurality of possibly overlapping operating rangeseach operating range covering a range of frequencies and adjustablewithin each range. One significant area of application is in signalrecorders and especially audio recorders. Other applications exist suchas plural range tone controls.

Due to the signal transfer characteristics of present audio recorders,and especially the characteristics of recording tape, the high frequencyrange of the audio signal becomes attenuated by the recording andreproduction process. This attenuation occurs to varying degreesdependent upon a number of variables including relative transducer torecord medium speed and the frequency characteristics of the tape.Consequently, an adjustable high frequency amplitude equalizationcircuit is commonly employed in the audio record circuitry of therecorder to compensate for the attenuation. The audio equalizationcircuitry may be designed into the audio preamplifier.

In the past, passive RC filters utilizing adjustable capacitors havebeen used with limited success as adjustable amplitude equalizers. Forexample, many multiple speed audio tape recorders have tape speedcapabilities ranging from 33/4 inches per second to 30 ips. Recordershaving a wide range of selectable operating speeds have required foreach speed a separate filter with a variable capacitor. While suchpassive equalizers effect equalization, they do so at the expense ofappreciable overall signal attenuation and concomitant loss of signalpower and quality. Because the filters require a larger number ofelements and a variable capacitor for each tape speed, the passiveequalizers are expensive.

It is preferable to utilize an active filter to minimize signalattenuation. The use, however, of a separate active filter for each tapespeed in a multiple speed machine can increase expenses beyond the costof using separate passive filters.

Many filter circuits, both passive and active, can introduce asignificant amount of noise into the signal. Many introduce noise whenthe equalization provided by the circuit is adjusted to zero. The samelevel of noise may be introduced regardless of the amount ofequalization provided. Active filters, moreover, may introduce noiseinto the system inversely to the level of signal amplitude enhancementor equalization introduced. This .[.later.]. .Iadd.latter .Iaddend.caseis especially undesirable because the signal to noise ratio becomes.[.high.]. .Iadd.low .Iaddend.with small amounts of equalization.

SUMMARY OF THE INVENTION

Accordingly, the present invention constitutes a high frequencyamplitude equalization circuit, preferably for the record circuitry of asignal recorder, that is adjustable to provide a desired signalfrequency response over a wide range of relative transducer to recordmedium speeds. The signal to be equalized is passed through two paths,one of which includes an active filter circuit. The active filtercircuit is responsive to the signal to vary with frequency the amplitudeof any of its frequency components lying in a predetermined frequencyrange. In one preferred embodiment, the active filter circuit isconstructed to vary the amplitude of the signal's frequency componentsin a high frequency range. The signal from the active filter circuit iscombined with the signal passed through the other of the two paths forfurther utilization. The active filter circuit comprises adifferentiating circuit, .Iadd.formed by .Iaddend.an amplifier.Iadd.circuit, .Iaddend.and a plurality of adjustable resistancesselectable one for each machine operating speed. Preferably, thedifferentiating circuit is .[.an RC circuit connected to the input ofthe amplifier.]. .Iadd.implemented by an operational amplifier having aninput RC series circuit and a feedback resistor cooperating to providethe differentiation.Iaddend.. The output of the amplifier, which isvarying with frequency, is adjusted by one of the plurality ofadjustable resistances selected according to desired speed thusproviding equalization over one of a plurality of operating or frequencyranges. The .[.RC differentiator and amplifier are.]..Iadd.differentiating circuit is .Iaddend.utilized for all operatingspeeds. Additional adjustable resistances only are needed for additionaloperating speeds. In the preferred embodiment, the filter circuit isplaced in parallel with the input resistance of .[.an.]. .Iadd.a summing.Iaddend.operational amplifier in the audio preamplifier circuit. Thus,the input signal follows two paths. In one path, the signal is feddirectly to the .Iadd.summing .Iaddend.amplifier through the inputresistor and in the other path, the signal is fed through the filtercircuit to the .Iadd.summing .Iaddend.amplifier. Preferably, theadjustable resistances are placed in parallel between the output of thefilter amplifier and ground. This arrangement causes a minimum of noiseto be introduced into the signal. When no high frequency equalization orboost is introduced into the circuit, i.e., when the tap of theadjustable resistance selected is turned to ground, no noise isintroduced. Other additional pre-emphasis circuits may be convenientlyadded to affect a low frequency amplitude boost or to provide othercompensation, if desired, by using the feedback path from the output ofthe summing amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of an audio preamplifier utilizingan embodiment of the equalization circuit of the present invention.

FIG. 2 is a graph representing the varying response curves availableutilizing the present invention.

FIG. 3 is a schematic circuit diagram of an alternative embodiment ofthe equalization circuit of the present invention utilizing additionalfilter pre-emphasis circuits.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, .[.1,.]. terminal 1 is the input to the audio preamplifiershown. Terminal 2 is the output. The main signal path of thepreamplifier comprises input resistor 4, amplifier 3, and feedbackresistor 5 connected between the amplifier input terminal 6 and theoutput terminal 2. The gain of the .Iadd.summing operational.Iaddend.amplifier 3 is, with little error: ##EQU1## Where A = theclosed loop gain of the operational amplifier.

R₅ = the resistance of feedback resistor 5

R₄ = the resistance of input resistor 4

Connected in parallel with input resistor 4 is an active filter circuit10, which is designed to perform a high frequency signal amplitudepre-emphasis. Any signal applied to input terminal 1 will thus passthrough input resistor 4 and the filter circuit 10. The resultantsignals will be summed at terminal 6 and then amplified by amplifier 3.The signal passing through the filter circuit is emphasized andthereafter summed with the signal passing through resistor 4 andamplified by amplifier 3 to form the desired equalized signal.

The active filter circuit 10 comprises .[.an.]. .Iadd.a differentiatingoperational .Iaddend.amplifier 11 with a feedback resistor 12. Connectedbetween terminal 1 and the input 24 of the amplifier 11 is a series RCnetwork comprising resistor 13 and capacitor 14. .Iadd.The capacitor 14and resistor 13 represent the input impedance of the differentiatingoperational amplifier 11. .Iaddend.A plurality of adjustable resistors15, 16 are connected between the output 25 of amplifier .[.12.]..Iadd.11 .Iaddend.and ground. There is one adjustable resistor for eachoperating speed. Connected respectively to the adjustable taps ofadjustable resistors 15, 16 are connecting resistors 18, 19.

A switch 20 selectively connects the .[.amplifier's.]. input terminal 6.Iadd.of summing amplifier 3 .Iaddend.to one of the connecting resistors18, 19. One terminal 21, 22, 23 of switch 20 is selected according tothe selected operating speed.

In operation, an audio signal is coupled to terminal 1. The signalpasses through the path formed by resistor 4 and also through the pathformed by resistor 13, capacitor 14 and amplifier 11 where it isdifferentiated and amplified. The differentiation of the signal causessignal amplitude in this embodiment to rise with frequency above apredetermined frequency as described below. The selection of thecomponent values of the active filter circuit determines the frequencyrange of the resultant high frequency pre-emphasis. In the preferredembodiment the output signal of amplifier 11 rises at 6 db/octave withincreasing frequency. The limit to this rise is set by R₁₃ C where R₁₃is the value of resistor 13 and C is the value of capacitor 14.

One terminal, for example 21, of switch 20, is selected according to achosen operating speed to connect adjustable resistor 15 to terminal 6by way of resistor 18. Accordingly, a portion of the differentiatedsignal dependent on the position of the tap of the adjustable resistoris summed with the undifferentiated signal, which has passed throughresistor 4 to terminal 6. The resultant signal is amplified byoperational amplifier 3 and available as an equalized output at terminal2.

.[.The frequency above which the signal is pre-emphasized by the activefilter circuit 10 is determined by:.]. ##EQU2## .Iadd.

The ratio of the output voltage to the input voltage for operationalamplifiers is given by the equation ##EQU3## Where V_(out) = the outputvoltage;

V_(in) = the input voltage;

Z_(fb) = the feedback impedance of the operational amplifier; and

Z_(in) = the input impedance of the operational amplifier.

For the differentiating operational amplifier 11, the equation is##EQU4## Where V₂₅ = the output voltage at terminal 25;

V₁ = the input voltage at terminal 1 of the equalization circuit;

R₁₂ = the resistance of feedback resistor 12;

R₁₃ = the resistance of input resistor 13; and

1/jwC = the impedance of input capacitor 14.

After rationalizing the equation (3) becomes: ##EQU5##

Equation (4) may be re-arranged as: ##EQU6##

In the frequency range over which signal pre-emphasis occurs, thecurrent from active filter 10 into the summing terminal 6 throughresistor 18 is equal to or greater than the current into the terminal 6from resistor 4. The differentiating operational amplifier 11 provides acurrent to the summing terminal 6 through a voltage divider formed bythe adjustable resistor 15 and through the resistor 18. The open circuitsource voltage at tap of resistor 15 with the load formed by resistor 18and following circuit at terminal 6 disconnected is,

    V.sub.T = KV.sub.25                                        (6)

where

V_(T) = the voltage at the tap of resistor 15;

K = the fraction of the resistance R₁₅ between the tap and ground.

The source impedance seen at the tap of resistor 15 as determined byThevenin's theorem is ##EQU7## Where R₁₅ = the resistance of adjustableresistor 15.

The current flowing into terminal 6 through resistor 18 from thedifferentiating operational amplifier 11 is ##EQU8## Where R₁₈ = theresistance of resistor 18.

At lower signal frequencies, wCR₁₃ is much less than 1 and equation (5)reduces to

    V.sub.25 = -jwCR.sub.12 V.sub.in                           (9)

The current I₁₈ at lower signal frequencies is obtained by substitutingequation (9) for V₂₅ in equation (8), which gives ##EQU9##

The current into summing terminal 6 through resistor 4 is

    I.sub.4 = V.sub.1 /R.sub.4                                 (11)

where

R₄ = the resistance of the resistor 4.

As discussed above, pre-emphasis occurs when the absolute magnitude ofthe current I₁₈ is equal to or greater than that of I₄, that is when

    |I.sub.4 | ≦ |I.sub.18 |(12)

after substitution for I₄ and I₁₈ from equations (11) and (10) intoequation (12) we obtain: ##EQU10##

After substituting for w=2π f and solving for the frequency f, the lowerfrequency limit f₁ at which pre-emphasis occurs can be found and isgiven by the equation: ##EQU11## where R₄ = the resistance of resistor 4

R₁₂ = the resistance of resistor 12

.[.R₁₃ = the resistance of resistor 13.].

.Iadd.R₁₅ = the resistance of resistor 15 .Iaddend.

R₁₈ = the resistance of connecting resistor 18

C = capacitance of capacitor 14

K = the fraction of the adjustable resistor 15 between the tap andground

f₁ = frequency.

The pre-emphasis frequency, then, is dependent on the positioning of thetap on the adjustable resistor 15. The lowest pre-emphasized frequencyis determined, when K = 1, by the values of R₄, R₁₂, R₁₈ and C. When thetap is set to ground, K = 0 and there is no pre-emphasis.

.[.The.]. .Iadd.As noted above, the limit of the 6db/octave rise in theoutput of amplifier 11 with increasing frequency is determined by CR₁₃.From equation (5), it is seen the frequency at which this occurs isdetermined by the denominator as wCR₁₃ approaches 1. Thus, the.Iaddend.upper frequency limit of signal pre-emphasis is determined by##EQU12## .Iadd.

Equation (4) also defines the phase relationship between the output andinput signals of the differentiating operational amplifier 11. Thisequation (4) can be rewritten in terms of real and imaginary componentsas ##EQU13##

From equation (16) it can be seen that the output voltage at terminal 25always lags in phase with respect to the input voltage at terminal 1.From equation (5), it can further be seen that the phase lag is 90° atfrequencies below f₂ where wCR₁₃ is much less than 1. .Iaddend.

It can be readily seen that where different amplitude equalization isrequired by different tape speeds, only the adjustable resistor 15 andthe connecting resistor 18 need be substituted with new values such asresistors 16 and 19. Resistors 12 and 13, capacitor 14 and amplifier 11are common to all speeds.

When there is zero boost or equalization, the tap of the adjustableresistor is set to ground and no output signal from the active filtercircuit is transferred to the main circuit path. Consequently, there isno noise contribution from the pre-emphasis circuit filter. When boostis provided, output noise remains substantially constant until K becomesclose to unity in which case some additional noise is contributed byamplifier 11 at high frequencies only. Thus a minimum amount of noise isintroduced into the circuit.

FIG. 2 shows a family of curves depicting the range of high frequencyequalization curves obtainable at a given tape speed as resistor 15, forexample, is adjusted. As resistor 15 is adjusted, frequency f₁ will varyas shown by the arrow. Frequency f₁ will vary from a minimum amount whenK=1 to higher amounts when K is reduced. The selection of anotheradjustable resistor 16 is necessary to provide a different range overwhich f₁ must vary due to different signal equalization required bymachine characteristics which change with operating speed. Along theordinate is plotted gain G, and along the abscissa is plotted frequencyf. The gain of amplifier 3 is shown at 30 and equals R₅ /R₄. The upper.Iadd.frequency .Iaddend.limit of boost 31 is given by the equation forf₂. The family of curves 32, 33, 34 represents the range of equalizationthat is obtained by varying K. The lowest possible corner frequency,when K=1, is shown at 35. As the corner frequency increases, the amountof signal amplitude boost .[.decrease.]. .Iadd.decreases .Iaddend.untilnone is present in the equalization.

Accordingly, the output of amplifier 3 consists of two components, onewhose amplitude is constant with frequency and one of which iscontributed by the pre-emphasis circuit and whose amplitude rises withfrequency over a selected range of frequencies.

Additional compensating networks may be added for adjustable lowfrequency pre-emphasis or to provide other compensation. The additionalnetworks are preferably switched using the same switch as utilized forhigh frequency pre-emphasis. As shown by way of example only in FIG. 3,additional networks 40, one or more for each operating speed, may beconnected between the switch terminals 21, 22, 23 and the circuit output2. The particular low frequency pre-emphasis circuit shown consists of acapacitor 41 in series with resistor 42. In parallel with resistor 42 isanother series RC circuit comprising capacitor 43 and resistor 44. Onesimilar circuit could be provided for each range of operation. Thecircuit, of course, need not take the form specifically described, butmay be of any suitable configuration to provide the desiredequalization.

The invention provides a high frequency equalization circuit in an audiopre-amplifier. The active filter circuit includes a series RC circuitconnected to the input of .[.an.]. .Iadd.a negative feedback.Iaddend.amplifier .Iadd.forming an operational amplifier .Iaddend.fordifferentiating the input signal to the preamplifier. The output of thefilter amplifier is connected to ground through a plurality of variableresistances connected in parallel. Each variable resistance provides ameans to vary the amount of high frequency boost utilized at one of aplurality of tape speeds of an audio tape recorder. Accordingly, one.[.RC differentiator and amplifier are.]. .Iadd.differentiatingamplifier is .Iaddend.utilized at all operating speeds requiring aminimum of component change for different speeds. Due to the describedcircuit configuration, there is a very low noise contribution to thepreamplifier from the active filter circuit except at close to maximumboost or pre-emphasis when the noise contribution is at high frequenciesonly. Since the variable resistances are grounded at one end, there isno noise contribution at zero boost when the taps on the resistances areset to ground. The input signal is passed both through the filter andthrough a resistor directly into the preamplifier where the sum of thetwo components is amplified. An alternative embodiment of the inventionincludes additional pre-emphasis circuits connected to be selectivelyswitched with the adjustable resistances to provide low frequency orother desired special equalization.

Although the invention has been described herein with reference to twoembodiments, it is to be understood that various modifications may bemade thereto within the spirit and scope of the invention. Thus it isnot intended to limit the invention except as defined in the followingclaims:

What is claimed is:
 1. An equalization circuit comprising:an input.Iadd.terminal.Iaddend.; amplifying means .Iadd.having an input.Iaddend.connected to said input .Iadd.terminal.Iaddend.; filter circuitmeans having an output .[.connected.]. .Iadd.for connection .Iaddend.tosaid input .Iadd.of said amplifying means and .Iaddend.for providing asignal varying in amplitude with frequency at its output; a plurality ofadjustable resistances corresponding to a plurality of operating rangesconnected to the output of said filter circuit means for adjusting thesignal amplitude provided by the output; and switch means forselectively connecting one of said plurality of adjustable resistancesto said .Iadd.input of said .Iaddend.amplifying means according to oneselected operating range.
 2. The circuit of claim 1 wherein:said filtercircuit means provides a signal which rises with frequency.
 3. Thecircuit of claim 1 wherein:said filter circuit means comprises an RCseries circuit .[.connect.]. .Iadd.connected .Iaddend.to said input.Iadd.terminal.Iaddend..
 4. The circuit of claim 1 wherein:said filtercircuit means comprises active filter means.
 5. The circuit of claim 1wherein:said filter circuit means comprises differentiating means.
 6. Anequalization circuit comprising:an input terminal; a first amplifierhaving an input and an output; an input resistance connected betweensaid input terminal and the input of said first amplifier; and, a filtercircuit connected in parallel with said input resistanceincluding:circuit means having an output to filter a signal received atsaid input terminal; a plurality of adjustable resistances connected tothe output of said filter circuit means, each of said resistancesselected according to a different one of a plurality of operating rangesfor adjusting the amplitude of the signal provided by said filtercircuit means; and, switch means connected between said plurality ofadjustable resistances and the input of said first amplifier forselectively connecting said adjustable resistances to said firstamplifier.
 7. A circuit of claim 6 wherein:said filter circuit meansprovides a signal varying with frequency.
 8. The circuit of claim 6wherein:said filter circuit means provides a signal rising withfrequency.
 9. The circuit of claim 8 wherein:said filter circuit meansincludes an RC circuit connected to said input terminal and a secondamplifier connected to said RC circuit, the output of said secondamplifier being connected to said plurality of adjustable resistances.10. The circuit of claim 9 wherein:said switch means comprises aplurality of contacts connected respectively to said plurality ofvariable resistances; and further including: a plurality of frequencypre-emphasis means connected respectively between said plurality ofcontacts and the output of said first amplifier.
 11. The circuit ofclaim 6 wherein:said filter circuit means comprises an RC series circuitconnected to said input terminal.
 12. The circuit of claim 6wherein:said filter circuit means comprises active filter means.
 13. Thecircuit of claim 6 wherein:said filter circuit means comprisesdifferentiating means. .Iadd.
 14. An equalization circuit, comprising:a. an input terminal for receiving an input signal to be equalized; b. afirst amplifying means having an input and an output; c. a first meansfor connecting said first amplifying means to said input terminal andproviding an output signal which is substantially in phase with saidinput signal; d. a phase shift circuit means coupled to said inputterminal and providing an output signal having an increasing amplitudeand constant phase lag with frequency relative to said input signal,within a predetermined frequency range of said input signal; e. a secondmeans for connecting the output of said phase shift circuit means tosaid input of said first amplifying means; and f. said first amplifyingmeans providing an output signal representative of the sum of saidrespective output signals provided by said first connecting means andsaid phase shift means. .Iaddend..Iadd.
 15. The circuit of claim 14wherein:said phase shift circuit means provides an output signalincreasing in amplitude and lagging in phase by 90°. .Iaddend..Iadd. 16.The circuit of claim 15 wherein: said phase shift circuit meanscomprises an inverting differentiating operational amplifier means..Iaddend. .Iadd.17. The circuit of claim 14 wherein said secondconnecting means comprises a plurality of variable resistances andswitch means for selectively connecting one of said plurality ofvariable resistances to the input of said first amplifying means..Iaddend. .Iadd.
 18. The circuit of claim 14 wherein said secondconnecting means comprises a resistive means. .Iaddend. .Iadd.
 19. Anequalization circuit comprising: a. an input terminal for receiving aninput signal to be equalized; b. a first amplifying means having aninput and an output, said input of the first amplifying means coupled tosaid input terminal to receive a signal which is substantially in phasewith said input signal; c. a second amplifying means having an input, anoutput, a feedback resistor connected between said output and input toprovide negative feedback, and a capacitive reactive impedance meanscoupled between said input terminal and the input of said secondamplifying means, said second amplifying means providing an outputsignal increasing in amplitude and having a constant phase lag withfrequency relative to input signal within a predetermined frequencyrange of said input signal; and d. means for connecting the output ofsaid second amplifying means to said input of said first amplifyingmeans. .Iaddend. .Iadd.
 20. The circuit of claim 19 wherein: saidconnecting means comprises a plurality of variable resistances; andswitch means for selectively connecting one of said plurality ofvariable resistances to the input of said first amplifying means..Iaddend..Iadd.
 21. The circuit of claim 20 wherein: said switch meansfurther comprises a plurality of contacts connected respectively to saidplurality of variable resistances; and further includinga plurality offrequency pre-emphasis means connected respectively between saidplurality of contacts and the output of said first amplifying means..Iaddend.